High-performance ceramic materials are essential for many engineering applications. Ultrafine ceramic powders are used to provide ceramic structures having the integrity, strength, and uniformity necessary to meet high performance requirements. To obtain many of the desirable properties associated with advanced ceramics, (e.g., toughness, high ductility, low sintering temperature and/or superplasticity) ceramic powders having nanometer scale crystalline grain structure, uniformity of crystalline phase, limited degree of particle aggregation, chemical purity, and narrow particle and grain size distributions are essential. Bowen, (1980) Mater. Sci. Eng., 44:1; Andres et al, (1989) J. Mater. Res., 4(3):704; Wakai and Nagano, (1991) J. Mater. Sci., 26(1):241. Large scale exploration of the properties of these materials has been limited, however, by a lack of large quantities of inexpensive nanophase powder having the desired properties.
To address this need, researchers have been exploring many approaches to nanophase powder production. For example, spray drying or spray pyrolysis has been used to produce unagglomerated crystalline powders. In this process, precursor salts are dissolved in water which is sprayed into a high temperature environment. Ceramic particles form as a result of nucleation in the liquid phase, driven by evaporation of water. Although chemical purity is high, processing rates are limited by the need to maintain very low concentrations of precursor solute in the droplets to prevent formation of undesirable fragments and cenospheres. Kodas et al., (1988) Appl. Phys. Lett., 52:1622; Zhang et al., (1990) J. Am. Ceram. Soc., 73(1):61.Industrial flame processes such as SiCl.sub.4 oxidation are inherently high rate processes. Such flame processes are best suited to single component systems such as TiO.sub.2 or SiO.sub.2, and may produce sintered agglomerated powders having a large mean particle diameter due to the extended residence time at high temperature. Ulrich and Riehl, (1982) J. Colloid Inter. Sci., 87:257. Further, ultrafine silica particles have been produced and deposited onto a fiber as part of a fiber optic waveguide manufacturing process. Morse, et al. SPIE, 1171-1207 (1989). None of the processes and devices described above meet the criteria for optimum production of nanophase ceramic powders, i.e., the high temperatures required for crystallinity and the short processing (residence) times required for minimal agglomeration.